Wilf,
Lifter pump up will occur, as you mentioned, at high RPMs when the lifter is actually "tossed" "up" by the cam lobe (over powering the valve spring which is supposed to hold everything tight). When the lifter loses contact with the lobe the
oil pressure within the lifter causes it to lengthen/expand. When the lifter regains normal contact/operation with the lobe it is now longer and may hold the valve open with resulting "backfire". The engine
oil pressure never gets so high that it forces the valve train to hold the valves open, except as described above.
Gerry A.,
The Comp Cams senario may be your answer, but I am in doubt. I don't have a "smarter" answer, but I'm not totally comfortable with their explanation either.
auto10x
Bill